This invention relates to electrostatic printing and more particularly to an improved ink and liquid developer composition for converting latent electrostatic images into visible images.
In electrostatic printing, latent electrostatic images are formed on a photoconductive surface of a recording element by uniformly charging the surface thereof, as by a corona discharge device, followed by exposure to light in the desired image pattern. Such images may be developed by solid (powder) developers or by liquid developers.
Liquid developer compositions for use with electostatic images generally comprise a dispersion of pigment or toner particles in a volatile, insulating liquid of high volume resistivity in excess of 10.sup.9 ohm-centimeters. Suitable insulating liquids include: aromatic hydrocarbons, such as benzene, toluene, and xylene; aliphatic hydrocarbons, such as, hexane, cyclohexane and heptane; freons and halogenated hydrocarbons and silicone oils. The liquid developer is applied to the imaged surface, and the suspended toner particles become electrostatically charged and develop the latent image by migration under influence of the image charge. This is known as electrophoretic development.
In another type of electrostatic image development disclosed by Gundlach in U.S. Pat. No. 3,084,043, liquid developers having relatively low viscosity, low volatility, contrast in color in the usual case with the surface on which it will remain, and relatively high electrical conductivity (relatively low volume resistivity), are disclosed for converting the electrostatic latent image to a visible image. According to this method liquid developer from a reservoir is deposited on a gravure roller and fills the depressions in the roller surface. Excess developer is removed from the lands between depressions, and as a receiving surface charged in image configuration passes against the gravure roller, developer is attracted from the depressions in image configuration by the charge. This method of development is referred to as polar liquid development.
Print quality is difficult to control when prior art water and alcohol based inks are used as liquid developers. Evaporation can be a problem with the volatile prior art liquid developers, and when a volatile, conductive liquid carrier or solvent is organic in nature, evaporation results in pollution of the surrounding air, a very undesirable charcteristic in view of the present day efforts to reduce or eliminate contaminants in the air.
The prior art liquid developers also are generally unsatisfactory from the standpoint of producing a permanent visible image which is free of smear or blurring especially when rubbed. This stems from the inability of the toner particles to become permanently fixed on the surface or in the fibers of the copy sheet. Other prior art liquid developers are somewhat hygroscopic and have a tendency to pick up moisture while standing and/or during application of the developer to the imaged surface thereby complicating the control of print quality and stability.
Certain non-epoxy ether-containing liquid developers in insulating carriers have been used in the development of electrostatic latent images. In U.S. Pat. No. 3,290,251 suitable ethers for use in insulating media include, for example, furane, dibutyl ether, diphenyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, ethylene glycol dibutyl ether, diethylene glycol diethyl ether and diethylene glycol dibutyl ether. However, these liquid developers are in a non-conductive, insulating medium, and therefore, they are not sufficiently conductive for use in the development of electrostatic latent images where the liquid developer is attracted to the image configuration without separation of the ingredients of the liquid developer. Furthermore, the developers disclosed therein are generally too volatile and tend to have varying compositions and properties due to evaporation. With these prior art compositions drying procedures must be used to permit evaporation of the solvent before the ink or developer becomes fixed to the substrate.